Fischer-tropsch catalyst with low surface area alumina, its preparation and use thereof

ABSTRACT

A catalyst for use in a Fischer-Tropsch synthesis reaction which comprises cobalt supported on alumina. The alumina support has a specific surface area of less than 50 m 2 /g and/or is at least 10% α-alumina.

[0001] The present invention relates to Fischer-Tropsch (F-T) catalysts,their use in F-T synthesis reactions, methods of their use and methodsof their manufacture.

[0002] Conversion of natural gas to liquid hydrocarbons (“Gas ToLiquids” or “GTL” process) is based on a 3 step procedure consistingof: 1) synthesis gas production; 2) synthesis gas conversion by FTsynthesis; and 3) upgrading of FT products (wax and naphtha/distillates)to final products such as naphtha, kerosene, diesel or other products,for example lube oil base.

[0003] Supported cobalt catalysts are the preferred catalysts for the FTsynthesis. The most important properties of a cobalt FT catalyst are theactivity, the selectivity usually to C5 and heavier products and theresistance towards deactivation. Known catalysts are typically based ontitania, silica or alumina supports and various metals and metal oxideshave been shown to be useful as promoters.

[0004] A recent series of papers by Iglesia et al. including“Selectivity Control and Catalyst Design in the Fischer-TropschSynthesis: Sites, Pellets and Reactors” 1993 , has given a descriptionof the reaction network leading to various hydrocarbon products and amethodology to optimize catalyst properties towards the desired heavyhydrocarbons. The maximum C₅+ selectivity is obtained by designingcatalyst pellets with optimum intraparticle diffusion resistance. Thisis achieved by increasing intraparticle diffusion resistance to thepoint where secondary chain building reactions of primary products(alpha-olefins) are maximized without inducing significant diffusionresistance on the reactants (H2, CO) because this will lead to poorselectivity. This principle is shown to be generally applicable on allthe supports mentioned above. By plotting different catalysts withdifferent physical properties (particle size, porosity, cobalt loading,cobalt dispersion) a typical “volcano plot” is generated and the maximumC₅+ selectivity is found for intermediate values of a parameter “χ”which is a function of the parameters mentioned above and is a measureof the intraparticle diffusion resistance at a given set of reactionconditions.

[0005] Definition of χ.

χ=R₀ ² øθ/r_(p)   (1)

[0006] where:

[0007] R₀=Catalyst particle radius (m)

[0008] ø=Catalyst porosity

[0009] θ=Catalytic site density (sites/m²)

[0010] r_(p)=average pore radius (m)

[0011] According to Iglesia the optimum value of χ for a typical set ofFT reaction conditions (200° C., 20 bar, H₂/CO=2.1, 50-60% conversion)is about 500-1000×10⁻¹⁶ m⁻¹, irrespective of the nature of the catalystsupport used. From the definition of χ it appears that any of theparameters involved (particle radius, porosity, pore radius or sitedensity) can be varied to achieve the desired value of χ. However, thisis somewhat misleading due to the known relationship between specificsurface area, pore radius and porosity (or specific pore volume). Byintroducing these relationships, it will be seen that χ can be describedby the particle size, the cobalt loading, the cobalt dispersion and theporosity. Thus, it can be seen that χ is actually independent of poreradius and site density and is determined only by the volumetrictransport parameter which is controlled solely by particle size, thecobalt loading, the cobalt dispersion and the porosity.

[0012] The following known equations are valid for an ideal cylindricalpore structure:

r_(p)=2V_(g)/S_(g)   (2)

V_(g)=ø/ρ_(p)   (3)

ρ_(p)=(1−ø)ρ _(s)   (4)

[0013] where

[0014] V_(g)=specific pore volume (cm³/g)

[0015] S_(g)=specific surface area (m²/g)

[0016] ρ_(p)=particle density (g/cm³)

[0017] ρ_(s)=material density (g/cm³)

[0018] The site density term in (1) (θ=Co sites/m²) can be expressed by:

θ=Co sites/m2 surface area=X_(Co)D_(Co)A/S_(g)M_(Co)   (5)

[0019] where

[0020] X_(Co)=Total Co concentration in catalyst (g_(Co)/g_(cat))

[0021] D_(Co)=Co dispersion (fraction of total Co exposed)

[0022] A=Avogadro number=6.23 10²³ atoms/mole

[0023] M_(Co)=Co molecular weight=58.9 g/mole

[0024] By combining equations (2)-(5) with (1) it can be shown that χcan be written as:

χ=R ₀ ² X _(Co) D _(Co) A(1−ø)ρ_(s)/2M _(Co)   (6)

[0025] It is apparent from (6) that χ actually is independent of poreradius and only depends on the volumetric density of sites in the freepore volume of the catalyst. It is also clear that due to the secondorder dependency on particle size, the easiest way of controlling χ isto vary the particle size.

[0026] If a cobalt catalyst is to be used in a fixed-bed type reactor itis necessary to use particle sizes of 1 mm or larger in order to avoidunacceptable pressure drop over the reactor. However, the value of χ isthen far too high to achieve optimum selectivity, due to high reactantdiffusion resistance. This can to a certain extent be addressed by theuse of so called eggshell or rim type catalysts where the active cobaltcontaining phase is located in a relatively thin region in the outershell of the support. However, in slurry type reactors, it is necessaryto employ much smaller particles, typically 10-100 μm. It is then easilyseen that it will be extremely difficult to achieve χ values in thedesired region. For example, a catalyst with 10 weight % cobalt loading,5% Co dispersion, 50% porosity and 50 micron particles will haveχ=13×10¹⁶ m⁻¹.

[0027] It should also be kept in mind that the parameters in eq. (6) cangenerally not be changed independently, i.e. the higher Co loading themore difficult it is to achieve a high dispersion. Moreover, the lowerthe porosity the more difficult it becomes to use a high cobalt loading.A combination of 20 weight % cobalt loading, 10% Co dispersion and 30%porosity gives a higher volumetric cobalt density than can be seen inany reference known to the applicants. The corresponding value of χ fora 50 μm particle (which is suitable for slurry reactor operation) willthen be 75×10¹⁶ m⁻¹, which is still far lower than the optimum valuetaught by Iglesia.

[0028] Thus, there is no apparent teaching for preparing highselectivity catalysts for use with small particle sizes, such as areencountered in slurry reactors.

[0029] The applicants have concluded a series of experiments toinvestigate the effect of χ on selectivity using a rhenium promotedcobalt on alumina support catalyst. These show only limited optimizationpotential by changing χ through changing the particle size. The resultsare shown in FIG. 1. FIG. 1 shows the effect χ on selectivity using 20%Co1% Re/γ-Al₂O₃ catalyst (8% dispersion, 60% porosity, average particlesize (microns): 46, 113, 225, 363, 638). Fixed bed reactor tests wereconducted at: 200° C., 20 bar, H₂/CO=2.1, 50-70% conversion, >24 h onstream. All data have been replicated 2 or more times.

[0030] Iglesia suggests that the C₅+ selectivity can be increased bydecreasing the relative density or reactivity of olefin hydrogenationvs. olefin readsorption sites. This effect is a direct consequence ofthe formulation of the reaction network. However, no guidance is givenas to how this change can be built into a real catalyst.

[0031] It is an object of the present invention to provide a F-Tcatalyst for use in slurry reactors with improved selectivity to C₅₊hydrocarbon.

[0032] One of the requirements of a catalyst for use in a slurry reactoris that the particles of catalyst should retain their structuralintegrity. Catalysts which are supported on titania are relatively weakand though encouraging results have been achieved from the point of viewof selectivity, there may be a tendency for titania-supported cobaltcatalysts to disintegrate upon prolonged use. Alumina has an inherentlystronger resistance against attrition and break-up of the catalystparticles than titania and is thus a more preferred support materialfrom the point of view of mechanical properties.

[0033] According to one aspect of the present invention, there isprovided a catalyst for use in a Fischer-Tropsch synthesis reactionwhich comprises cobalt supported on alumina, in which the aluminasupport has a specific surface area of <50 m²/g preferably <30 m²/g, butpreferably not below 5 m²/g.

[0034] Preferably, the alumina is at least 50% alpha-alumina, with theremainder being gamma- and/or theta-alumina, preferably, predominantlytheta-alumina. Preferably, it is at least 80% or even substantially purealpha-alumina.

[0035] Preferably, the cobalt represents from 3 to 35% by weight of thecatalyst, more preferably from 5 to 20% by weight. The catalyst may alsoinclude up to 2% by weight of rhenium, e.g. 0.25 to 1% or 0.25 to 0.5%rhenium. Other known metallic promoters/dopants such as platinum,rhodium, iridium and palladium may also be included, preferably at thesame levels, as well as oxide promoters/dopants such as rare earthoxides and alkali metal oxides.

[0036] According to another aspect of the present invention, there isprovided a method of manufacturing a Fischer-Tropsch catalyst whichcomprises heat treating alumina particles at a temperature in the rangeof 700 to 1300° C. for a period of between 1 and 15 hours; andimpregnating the heat treated alumina particles with cobalt and anydesired promoters/dopants. Preferably, the treatment temperature is inthe range of 900 to 1200° C. and the treatment period is between 5 and10 hours.

[0037] The invention also extends to the use of a catalyst according tothe first aspect of the invention in a F-T synthesis reaction. This maysuitably be carried out in a slurry bubble column reactor.

[0038] The invention also extends to a method of converting natural gasto C₅+ hydrocarbons, which comprises; subjecting a natural gas feedstream to a reforming reaction to produce a synthesis gas feed stream ofhydrocarbon and carbon monoxide; subjecting the synthesis gas feedstream to a Fischer-Tropsch synthesis reaction in the presence of acatalyst according to the first aspect; and separating a product streamincluding C₅+ hydrocarbons.

[0039] The method of depositing the active metal, the metallicpromoters, the alkali and the rare earth oxide on the alumina support isnot critical, and can be chosen from various methods well known to thoseskilled in the art. One suitable method that has been employed is knownas incipient wetness impregnation. In this method the metal salts aredissolved in an amount of a suitable solvent just sufficient to fill thepores of the catalyst. In another method, the metal oxides or hydroxidesare coprecipitated from an aqueous solution by adding a precipitatingagent. In still another method, the metal salts are mixed with the wetsupport in a suitable blender to obtain a substantially homogenousmixture. In the present invention, if incipient wetness impregnation isused, the catalytically active metal and the promoters can be depositedon the support using an aqueous or an organic solution. Suitable organicsolvents include, for example, acetone, methanol, ethanol, dimethylformamide, diethyl ether, cyclohexane, xylene and tetrahydrofuran.

[0040] Suitable cobalt compounds include, for example, cobalt nitrate,cobalt acetate, cobalt chloride and cobalt carbonyl, with the nitratebeing the most preferable when impregnating from an aqueous solution.Suitable rhenium compounds include, for example, rhenium oxide, rheninumchloride and perrhenic acid. Perrhenic acid is the preferred compoundwhen preparing a catalyst using an aqueous solution. Suitable platinum,iridium and rhodium compounds include, for example, mitrates, chloridesand complexes with ammonia. Suitable alkali salts for incorporating thealkali into the catalyst include, for example, the nitrates, chlorides,carbonates, and hydroxides. The rare earth oxide promoter can suitablybe incorporated into the catalyst in the form, for example, of thenitrate or chloride.

[0041] After aqueous impregnation, the catalyst is dried at 110° C. to120° C. for 3 to 6 hours. When impregnating from organic solvents, thecatalyst is preferably first dried in a rotary evaporator apparatus at50° C. to 60° C. under low pressure, then dried at 110° C. to 120° C.for several hours longer.

[0042] The dried catalyst is calcined in air by slowly increasing thetemperature to an upper limit of between 200° C. and 500° C., preferablybetween 250° C. and 350° C. The rate of temperature increase ispreferably between 0.5° C. and 2° C. per minute, and the catalyst isheld at the highest temperature for a period of 1 to 24 and preferably 2to 16 hours. The impregnation procedure is repeated as many times asnecessary to obtain a catalyst with the desired metals content. Cobalt,rhenium, alkali and the rare earth oxide promoter, if present, can beimpregnated together, or in separate steps. If separate steps are used,the order of impregnating the active components can be varied.

[0043] Before use, the calcined catalyst is preferably reduced withhydrogen. This can be suitably carried out by flowing hydrogen at aspace velocity of at least 1000 Ncm³/g. The temperature is slowlyincreased from ambient to a maximum level of 250° C. to 450° C.,preferably between 300° C. and 400° C., and maintained at the maximumtemperature for about 1 to 24 hours, more preferably 5 to 16 hours.

[0044] The reactor used for the synthesis of hydrocarbons from synthesisgas can be chosen from various types well known to those skilled in theart, for example, fixed bed, fluidized bed, ebullating bed or slurry.The catalyst particle size for the fixed or ebullating bed is preferablybetween 0.1 and 10 mm and more preferably between 0.5 and 5 mm. For theother types of operations a particle size between 0.01 and 0.2 mm ispreferred.

[0045] The synthesis gas is a mixture of carbon monoxide and hydrogenand can be obtained from any source known to those skilled in the art,such as, for example, steam reforming of natural gas or partialoxidation of coal. The molar ratio of H₂:CO is preferably between 1:1 to3:1; and more preferably between 1.5:1 to 2.5:1. Carbon dioxide is not adesired feed component for use with the catalyst of this invention, butit does not adversely affect the activity of the catalyst. All sulfurcompounds must, on the other hand, be held to very low levels in thefeed, preferably below 100 ppb.

[0046] The reaction temperature is suitably between 150° C. and 300° C.,and more preferably between 175° C. and 250° C. The total pressure canbe from atmospheric to around 100 atmospheres, preferably between 1 and50 atmospheres. The gaseous hourly space velocity, based on the totalamount of synthesis gas feed, is preferably between 100 and 20,000 cm³of gas per gram of catalyst per hour; and more preferably from 1000 to10,000 cm³/g/h, where gaseous hourly space velocity is defined as thevolume of synthesis gas (measured at standard temperature and pressure)fed per unit weight of catalyst per hour.

[0047] The reaction products are a complicated mixture, but the mainreaction can be illustrated by the following equation:

nCO+2nH₂→(—CH₂-)n+nH₂O

[0048] where (—CH₂—)_(n) represents a straight chain hydrocarbon ofcarbon number n. Carbon number refers to the number of carbon atomsmaking up the main skeleton of the molecule. In F-T synthesis, theproducts are generally either paraffins, olefins, or alcohols. Productsrange in carbon number from one to 50 or higher.

[0049] In addition, with many catalysts, for example, those based oniron, the water gas shift reaction is a well known side reaction:

CO+H₂O→H₂+CO₂

[0050] With cobalt catalysts the rate of this last reaction is usuallyvery low.

[0051] The hydrocarbon products from Fischer-Tropsch synthesis aredistributed from methane to high boiling compounds according to the socalled Schulz-Flory distribution, well known to those skilled in theart. The Schulz-Flory distribution is expressed mathematically by theSchulz-Flory equation:

W_(n)=(1−α)² nα ^(n-1)

[0052] where n represents carbon number, α is the Schulz-Florydistribution factor which represents the ratio of the rate of chainpropagation to the rate of chain propagation plus the rate of chaintermination, and W_(n) represents the weight fraction of product ofcarbon number n. This equation shows that an increased α results in ahigher average carbon number of the products. Higher α values aredesirable when heavier products, such as diesel fuel, are relativelymore valuable than lighter products, such as naphtha and light gases.

[0053] The invention is therefore concerned with the preparation and usein FT synthesis of a cobalt supported catalyst on low surface areaalumina for optimizing C₅+ selectivities. This is preferably achieved byheat treatment of high surface area aluminas to achieve the desiredsurface areas, but it is understood that any means of achievingmaterials with such properties are covered by the invention. Anotheradvantage of the invention is the surprisingly high activity and highresistance towards deactivation of the described materials.

[0054] The invention describes catalytic materials that can be used inany type of FT reactor that is suitable for synthesis of heavyhydrocarbons (e.g. fixed-bed and slurry reactors). It should beunderstood that any combination of cobalt and suitable promoters (suchas Re, Pt or other suitable components) will benefit from the use of thelow surface area alumina supports, including unpromoted cobaltcatalysts.

[0055] The catalysts of the invention provide a way to achieve high C5+selectivities at low values of χ, i.e. at low values of intraparticlediffusion resistance. Thus, these catalysts circumvent the limitationsimposed by the teaching of Iglesia. It has been discovered that cobaltsupported on low surface area alumina can achieve substantially improvedC5+ selectivities in FT synthesis compared to high surface area alumina,even at low values of χ. This has been achieved by heat treatment ofhigh surface area aluminas to achieve the desired surface areas. Theresults of tests conducted indicate that the increase in C5+ selectivitymay be at least partially attributable to a reduced olefin hydrogenationactivity relative to the main FT synthesis activity.

[0056] It has also been discovered that these catalysts, in spite of thelow surface area available for impregnation of active components have anactivity which is higher than comparable high surface area (HSA)catalysts at conditions which simulate high conversion in a slurrybubble column reactor (i.e. at high and uniform water partialpressures). In fact, the low surface area catalyst activity is close tothe activity of a high surface area catalyst with higher Co loading.

[0057] In the low surface area catalyst, the loss of activity per unittime is not affected but there is a reversible step change upwards inactivity for the low surface area catalyst which is not observed for theHSA catalyst.

[0058] A further benefit over known technology is that since thecomposition of the wax is displaced towards the heavy side (higher avalue), this leads to an increase in the middle distillate yield or lubeoil base when the wax is hydrocracked or hydroisomerised in a downstreamprocess. The consequence of this in a total GTL process is that therecycle of unconverted gas back to the natural gas reforming section canbe reduced, the overall efficiency of the process can be increased (i.e.CO₂ emissions will be reduced) and the oxygen consumption can bereduced. Still further, it has been discovered that the catalystsaccording to the invention show a reduced water-gas-shift activity,leading to decreased undesired CO₂ production.

[0059] The invention may be carried into practice in various ways andwill now be illustrated by the following examples.

[0060] In the drawings:

[0061]FIG. 1 is a graph showing the effect of χ on C₅+ selectivity;

[0062]FIG. 2 is a graph showing the effect of support surface area onC₅+ selectivity;

[0063]FIG. 3 is a graph showing C5+ selectivity as a function of % αAl₂O₃ in the support;

[0064]FIG. 4 is a graph showing the effect of χ on C5+ selectivity usingAl₂O₃ supported Co catalysts;

[0065]FIG. 5 is a graph showing the effect of cobalt loading on C₅+selectivity using Al₂O₃ supported cobalt catalysts;

[0066]FIG. 6 is a graph showing the effect of cobalt loading on catalystproductivity using Al₂O₃ supported cobalt catalysts;

[0067]FIG. 7 is a graph showing propene and propane selectivity as afunction of support surface area; and

[0068]FIGS. 8 and 9 are graphs showing respectively propane and propeneselectivity as a function of χ for Al₂O₃ supported Co catalysts.

EXAMPLE 1 Catalyst Preparation

[0069] The catalysts were prepared as follows: A solution was preparedby dissolving a given amount of cobalt nitrate, Co(NO₃)₂.6H₂O and insome of the catalysts also perrhenic acid, HReO₄ or tetra amin platinumnitrate, Pt(NH₃)₄(NO₃)₂ in a given amount of distilled water. The totalsolution was added with stirring to a given amount of Condea PuraloxSCCa 45/190 alumina treated in air at different temperatures prior toimpregnation, and the amount of solution added to the alumina wassufficient to achieve incipient wetness. The prepared catalysts weredried for 3 hours in an oven at a temperature of 110° C. The driedcatalysts were then calcined in air by raising its temperature at aheating rate of 2°/minute to 300° C. and holding at this temperature for16 hours. After calcination the catalysts were screened to the desiredparticle size. The amounts used in preparation and the content of theprepared catalysts are given in table 1a. TABLE 1a Catalyst preparationdata of Al₂O₃ supported catalysts. Catalysts written in bold areaccording to the invention. Other materials are included for comparison.Co(NO₃) Pt(NH₃)₄ Al₂O₃**** Catalyst Particle 6H₂O HReO₄ (NO₃)₂ WaterAl₂O₃ treatment composition size Cat. (g) (g) (g) (ml) (g) (° C.) (%)(microns)  1 1077.93 17.93 1 070  826*** 500 20% Co-1% Re 38-53  2 17.180.23 30   25 500 12% Co- 53-75 0.5% Re  2b 11.49 — 21   17 500 12% Co53-90  2c 133.39 —  108 500 20% Co 53-90  2d 130.83 2.17  105 500 20%Co-1% Re 53-90  3 16.93 0.23 22   25 1100 12% Co- 53-90 0.5% Re  4 11.400.16 12   17 1150 12% Co- 53-90 0.5% Re  4b 16.95 — 18   25 1150 12% Co53-90  5 2.61 0.04 7   10 1150 5% Co- 53-90 0.25% Re  6 4.31 0.08 7   101150 8% Co-0.4% Re 53-90  7 5.52 0.09 7   10 1150 10% C0- 53-90 0.5% Re 8* 62.68 1.05 38   50 1150 20% Co-1% Re 53-90  9 n.a. n.a. n.a. 10000500 20% Co- 38-53 10 1% Re**  75-150 11 150-300 12 300-425 13 425-85013b 10.16 0.14 9   15 1150 12% Co-  75-150 0.5% Re 13c 10.16 0.14 9   151150 12% Co- 150-300 0.5% Re 13d 10.16 0.14 9   15 1150 12% Co- 425-8500.5% Re 14 16.89 — 0.17 18   25 1150 12% Co- 53-90 0.3% Pt 15 16.89 —0.17 30   25 500 12% Co-0.3% Pt 53-90 16 271.41 3.73 340  400 1130 12%Co- 38-53 0.5% Re

[0070] Catalysts 9-13 are different particle sizes of the same catalyst,of which the particle sizes are made by tabletising the powder beforecrushing and screening. The catalyst (2×5 kg) was prepared by incipientwetness in a mixer, drying at 120° C. for 2 hours and calcining at 300°C. for 3 hours.

EXAMPLE 2 Cobalt Catalysts Supported on High Surface Area Alumina WithVarying Particle Size

[0071] Catalysts 9-13 in Table 1 were tested in an isothermal fixed-bedmicroreactor. The reactor was 25 cm long and had an inner diameter of 1cm. Each catalyst was given a pretreatment consisting of reduction bypassing hydrogen over the catalysts while heating the catalyst at a rateof 1° C./minute to 350° C. and maintaining this temperature for 16 hoursat a pressure of 1 bar. In the tests, synthesis gas consisting of 2.1:1H₂:CO (+3 vol % N₂) was passed over 1-2 g of the catalyst diluted 1:5with SiC at 20 bar at the desired temperature and space velocity. Thespace velocity was usually varied to keep the CO conversion between 40and 70%. Products from the reactor were sent to a gas chromatograph withFID and TCD detectors for analysis, and methane analysed on bothdetectors was used as a link in the calculations.

[0072] In order to investigate the influence of χ on C5+ selectivity,catalysts 9-13 were tested under the same reaction conditions asemployed by Iglesia et al.

[0073] The results are given in Table 1b and illustrated graphically inFIG. 1 and compared to the results of Iglesia et al. FIG. 1 shows theeffect of χ on C5+ selectivity using 20% Co1% Re-1RE/γ-Al₂O₃ catalyst(8% dispersion, 60% porosity, average particle size (microns): 46, 113,225, 363, 638).

[0074] The sharp decrease in C5+ selectivity at χ-values above ca.1000·10¹⁶ m⁻¹ is caused by intraparticle diffusion limitations for H₂and CO, as explained by Iglesia et al. However, in the present contextit is more important to notice that the C5+ selectivity of high surfacearea alumina-supported catalysts can not be increased significantly byvariation of χ(particle size) from low (<100·10⁶ m⁻¹) to intermediatevalues (500-1000·10¹⁶ m⁻¹) and other methods are thus evidently neededto increase the C5+ selectivity of alumina supported Co catalysts. TABLE1b Properties and results from catalytic tests of materials described intable 1. Reaction conditions: Fixed-bed reactor at 200° C., 20 bar, feedH₂/CO = 2.1, 50-70% conversion, >24 hrs onstream Support Mean treatmentAlumina Surface Co particle χ CO Reaction Selectivity¹⁾ Compositiontemp. phase area dispersion size m⁻¹ GHSV²⁾ conv. rate³⁾ (%) Cat. (wt %)(° C.) (% α) (m²/g) Porosity (%) (microns) (× 10¹⁶) (h⁻¹) (%) (g/g/h)CH₄ C₂-C₄ C₅+  9 20% Co-1% 500 0 182 0.60 8  46 29 2650 57 0.3 6 10.483.5 Re*⁾ 10 20% Co-1% 500 0 182 0.60 8 112 177 2380 57 0.27 6.1 10.883.1 Re*⁾ 11 20% Co-1% 500 0 182 0.60 8 225 707 2500 62 0.31 5.9 9.884.2 Re*⁾ 12 20% Co-1% 500 0 182 0.60 8 363 1836 2750 63 0.34 7.5 9.982.7 Re*⁾ 13 20% Co-1% 500 0 182 0.60 8 638 5678 3300 53 0.35 12.9 8.878.3 Re*⁾

EXAMPLE 3 Cobalt Catalysts Supported on Alumina With Different SurfaceArea and Phase Composition

[0075] Alumina supports with different surface area and alumina phasecomposition were prepared by heat treatment at different temperatures asdescribed in Example 1. The catalysts also contained varying amounts ofcobalt and promoters. The catalysts were tested in a fixed-bed reactorusing the same equipment and procedures as described in Example 2. Theresults for all of the catalysts are shown in Table 2 and illustrated inFIGS. 2, 3 and 4.

[0076]FIGS. 2 and 3 show the C5+ selectivity for all of the catalystswith χ<150·10¹⁶ m⁻¹ (i.e. all catalysts with small particles) as afunction of support surface area or a alumina content. Although there issome apparent spread in the data, it is quite clear that the low surfacearea/high α-alumina catalysts show significantly higher C5+selectivities than high surface area γ-alumina supported catalysts. Itis also evident that the effect is more significant at surface areasbelow ca. 50 m2/g and α-alumina content above ca. 10%.

[0077] Note also that the Schulz-Flory growth parameter (α) is increasedfor catalysts using low surface area alumina with a high content ofα-alumina (see catalyst 2,3 and 4 in Table 2). The increase from in afrom 0.92 to 0.94 gives an increase in wax (C19+) yield (in % of thetotal hydrocarbon production) of more than 10% units (from below 50% toabove 60%).

[0078]FIG. 4. shows a plot of C5+ selectivity as a function of χ forcatalysts from Table 2. It is evident that two parallel curves arisefrom the data, one for high surface area γ-alumina supports and anotherfor low surface area alumina with a high content of α-alumina. Thelatter shows on the average 4-6 % units higher C5+ selectivity than theformer for all values of χ. The apparent spread in data in FIGS. 2-4will be further explained by Examples 4 and 5. TABLE 2 Properties andresults from catalytic tests of materials described in table 1. Reactionconditions: Fixed-bed reactor at 210° C., 20 bar, feed H₂/CO = 2.1,40-70% conversion, about 100 hrs on stream Support Co treat- Alu- dis-Mean Reac- ment mina Surface per- particle χ CO tion Selectivity¹⁾Composition temp. phase area sion size m⁻¹ GHSV²⁾ conv. rate³⁾ (%) Cat.(wt %) (° C.) (% α) (m²/g) Porosity (%) (microns) (× 10¹⁶) (h⁻¹) (%)(g/g/h) CH₄ C₂-C₄ C₅+ α⁴⁾ α 1 20% Co-1% Re 500 0 183 0.65 8.3 46 27 710043 0.61 8.8 10.1 81.1 — 2 12% Co-0.5% Re 500 0 191 0.75 11.2 64 30 510049 0.50 9.1 9.2 81.8 0.92 2b 12% Co 500 0 191 0.75 9.8 72 33 4700 460.43 9.4 10.1 80.9 — 2c 20% Co 500 0 191 0.75 7.5 72 43 6200 45 0.55 9.710.8 79.5 — 2d 20% Co-1% Re 500 0 191 0.75 10.5 72 43 8400 46 0.77 9.39.9 81.2 — 3 12% Co-0.5% Re 1100 7 66 0.64 12.4 72 60 5500 50 0.54 8.48.7 83.0 0.92 4 12% Co-0.5% Re 1150 86 16 0.24 10.2 64 83 3900 55 0.436.8 5.4 87.8 0.94 4a 12% Co-0.5% Re 1150 86 16 0.24 10.2 84 139 4300 530.45 6.0 5.3 88.7 — 4b 12% Co 1150 86 13 0.19 6.8 72 74 3100 48 0.30 8.28.0 83.8 — 5  5% Co- 1150 86 13 0.19 8.5 72 38 1700 45 0.15 7.5 6.3 86.2— 0.25% Re 6  8% 1150 86 13 0.19 8.6 72 62 2900 45 0.26 6.6 5.4 88.0 —Co-0.4% Re 7 10% Co-0.5% Re 1150 86 13 0.19 9.6 72 87 3700 48 0.36 6.86.1 87.1 — 8 20% Co-1% Re 1150 86 16 0.24 5.7 72 98 4600 47 0.43 7.6 6.985.5 — 9 20% Co-1% Re*⁾ 500 0 182 0.60 8 46 29 4800 54 0.51 6.8 9.4 83.8— 10 20% Co-1% Re*⁾ 500 0 182 0.60 8 112 177 3800 55 0.41 7.1 9.7 83.1 —11 20% Co-1% Re*⁾ 500 0 182 0.60 8 225 707 4500 56 0.49 7.1 8.1 84.8 —12 20% Co-1% Re*⁾ 500 0 182 0.60 8 363 1836 5000 62 0.62 10.4 8.1 81.5 —13 20% Co-1% Re*⁾ 500 0 182 0.60 8 638 5678 4500 56 0.50 16.8 8.6 74.6 —13b 12% Co-0.5% Re 1150 86 7 0.11 7 113 207 3000 52 0.31 6.9 4.8 88.3 —13c 12% Co-0.5% Re 1150 86 6 0.11 6.6 225 781 3000 49 0.29 6.7 4.9 88.4— 13d 12% Co-0.5% Re 1150 86 7 0.08 7.7 613 6978 3400 49 0.34 13.4 680.6 — 14 12% Co-0.3% Pt 1150 86 13 0.19 7.7 72 84 3400 51 0.34 7.3 6.885.9 — 15 12% Co-0.3% Pt 500 0 191 0.75 8.8 72 30 4300 46 0.39 10.3 10.478.3 —

EXAMPLE 4 The Effect of Cobalt Loading on Alumina With Different SurfaceArea and Phase Composition

[0079] The tests in Example 4 were fixed bed reactor tests at: 210° C.,20 bar, H₂/CO=2.1, 45-55% conversion, ca. 100 h on stream.

[0080] The results indicate that there is an optimum loading of cobaltfor a given alumina surface area. A more thorough examination of theresults in Example 3 shows that some of the low C5+ selectivities forlow surface area/high α-alumina supports are caused by too high loadingof cobalt. This is illustrated in FIGS. 5 and 6. FIG. 5 shows the effectof cobalt loading on C5+ selectivity and FIG. 6 the effect on catalystproductivity using Al₂O₃ supported cobalt catalysts with differentsurface area/α-alumina content.

[0081] At 20% Co loading, there is a smaller gain in C5+ selectivity byusing low-surface area/high α-alumina support (FIG. 5). This is alsoclearly shown by examining the influence of Co loading on catalystactivity, as illustrated by the hydrocarbon production rate at thesereaction conditions (FIG. 6). In spite of the much lower surface areaand pore volume of the catalysts according to the invention, the cobaltutilisation is as good as for high surface area supports up to about 12%Co, after which it is apparent that the support can not effectivelydisperse the additional active metal.

[0082] However, the results are not intended to limit the invention toCo loadings below 12%, but merely to illustrate that there is an optimumlevel for each set of support properties. It is well known that theaccommodation of active metal in supports can be varied and optimised bythe method of impregnation, the type of cobalt precursor, the solventused, the number of impregnation steps and the conditions forpretreatment of the catalyst to mention only a few.

EXAMPLE 5 The Effect of Metal Promoters

[0083] Although the results show a marked effect of surfacearea/α-alumina content for all of the catalysts, it is clear that thereis synergy between the use of a metal promoter such as Re or Pt and thesupport properties. This is illustrated in Table 3, showing that theeffect of low surface area/high α-alumina supports is clearly larger forthe Pt and Re promoted catalysts compared to the unpromoted catalyst.

[0084] In order to make sure that the observed effect of promoters werenot caused by secondary factors (χ) through the higher activity(dispersion) of these catalysts, experiments with Re promoted catalystswith lower Co loading and thus lower activity (and χ) were alsoperformed. The results are given in Table 4, showing the positive effectof Re for catalysts with virtually constant activity (and χ-value).TABLE 3 Difference in C5+ selectivity (ΔC5+) between low surfacearea/high α- alumina and high surface area/γ-alumina based Co catalysts,with and without promoter. Catalyst sample numbers refer to Tables 1 and2. Fixed bed reactor tests at: 210° C., 20 bar, H₂/CO = 2.1, 45-55%conversion, ca. 100 h on stream). Catalysts (no.) ΔC5+ (%) 12Co (2b/4b)3.5 12Co0.5Re (2/4) 6.0 12Co0.3Pt (15/14) 7.6

[0085] TABLE 4 Reaction rate and C5+ selectivity for catalysts supportedon low surface area/ high α - alumina with near-constant χ. Catalystsample numbers refer to Table 1 and 2. Fixed bed reactor tests at: 210°C., 20 bar, H₂/CO = 2.1, 45-55% conversion, ca. 100 h on stream).Select. Composition χ Reaction rate C₅+ Cat. (wt %) (m⁻¹ × 10¹⁶)(gHC/gcat/h) (%)  4b 12% Co 74 0.30 83.8 4 12% Co - 0.5% Re 83 0.43 87.86  8% Co - 0.4% Re 62 0.26 88.0 7 10% Co - 0.5% Re 87 0.36 87.1

EXAMPLE 6 Water-Gas Shift Activity

[0086] The water gas shift reaction (CO+H₂O=CO₂+H₂) is generally anunwanted side reaction to the main hydrocarbon synthesis formation. Thewater gas shift activity of the catalysts was tested by adding water(steam) to the feed in fixed-bed catalyst testing experiments otherwisesimilar to the experiments described in Example 2. This has theadvantage that the water partial pressure is higher and more uniformover the reactor and thus facilitates interpretation of the data.

[0087] Typical results for catalysts with low surface area/highα-alumina and high surface area/γ-alumina are shown in Table 5.

[0088] Although most cobalt catalysts have relatively low water gasshift activity, the results show that the catalysts according to theinvention have still significantly lower (a factor of 2) CO₂ formationrates compared to catalysts supported on high surface area/-γ-alumina.TABLE 5 CO₂ selectivity for low surface area/high α-alumina and highsurface area/γ-alumina based Co-Re catalysts. Catalyst sample numbersrefer to Table 1 and 2. Fixed bed reactor tests at: 210° C., 20 bar.Feed composition (molar): 50.5% H₂, 24% CO, 22-23% H₂O, balance N₂),40-50% conversion, 100-200 h on stream). Support CO₂ surface CO₂formation Composition area % select. rate Cat. (wt %) (m²/g) α-Al₂O₃ (%)(mmole/g_(cat)/h) 2 12% Co-0.5% Re 191 0 0.56 0.156 4 12% Co-0.5% Re 1686 0.28 0.087

EXAMPLE 7 Slurry Reactor Experiments

[0089] A catalyst according to the invention was also tested in slurryreactor in order to verify the selectivity advantage also under theconditions typical of such reactors. Results are shown in Table 6.

[0090] At virtually identical reaction conditions, the low surfacearea/high α-alumina supported catalyst show almost 7% increase in C5+selectivity(compared to a typical high surface area/γ-alumina basedcatalyst), which is even more significant than found in the fixed-bedreactor tests. The slurry reactor tests also confirm the difference inCO2 selectivities as described in Example 6. TABLE 6 Results from testsof Al₂O₃ supported Co catalysts (38-53 micron particles) in a 2 Lstirred slurry reactor (CSTR). T = 220° C., P = 20 bar, feed H₂/CO =2.0, 3% inerts (N₂) in feed. Results after > 100 h on stream. CatalystCO₂ surface CO Selectivity (% C, CO₂ Selec- area conv. free basis)tivity Catalyst (m²/g) (%) CH₄ C₂-C₄ C₅+ (%) 20% Co-1% Re- 140 77.4 8.37.8 83.9 2.7 1% RE* 12% Co-0.5% Re  25 77.7 5.3 3.8 90.8 1.1

EXAMPLE 8 The Effect of Water

[0091] The following example will illustrate that the positive influenceof the invention on C5+ selectivity is not dependent on the level ofwater concentration (steam partial pressure) in the reactor. Water is aproduct of the Fischer-Tropsch reaction and its partial pressure in thereactor will therefore be dependent on the conversion level. Thefollowing experiments were carried out in order to investigate theeffect of conversion on selectivity for a catalyst representative of theinvention and a comparative sample. In addition, experiments withaddition of water (steam) to the reactor were carried out to furtherprobe the effect of water. The experiments were carried out in afixed-bed reactor using the same experimental procedures as described inExample 2, apart from the addition of water and the deliberate variationof space velocity to influence conversion levels. The results are shownin Table 7.

[0092] It is evident that the effect of using low surface area/highα-alumina supports is independent on water partial pressure. TABLE 7 Theeffect of water partial pressure on C5+ selectivity for low surfacearea/high α-alumina and high surface area/γ-alumina based Co—Recatalysts. Catalyst sample numbers refer to Table 1 and 2. Fixed bedreactor tests at: 210° C., 20 bar, H₂/ CO = 2.1, 500-600 h on stream.Inlet Average CO H₂O H₂O conver- partial partial C₅+ ΔC₅+ Cat. Catalystsion pressure pressure selectivity Selectivity¹⁾ No. descr. (%) (bar)(bar) (%) (%) 4 12% Co- 24 0 0.9 86.4 5.4 0.5% Re 16 m²/g 50 0 2.2 88.44.6 86% α- 76 0 4.2 90.0 5.3 alumina 30 4.6 5.8 91.4 5.6 2 12% Co- 21 00.8 81.0 — 0.5% Re 191 m²/g 50 0 2.2 83.8 — 0% α- 74 0 4.0 84.7 —alumina 22 4.6 5.3 85.8 —

EXAMPLE 9 Olefin Hydrogenation Activity

[0093] Iglesia et al. have shown that both olefins and paraffins areprimary products of the FT reaction and that secondary hydrogenation ofolefins is an undesired side reaction, because olefins are thenprevented from further chain growth. A reduction in olefin hydrogenationactivity without decreasing the main hydrocarbon productivity wouldtherefore be a desired catalyst property. However, there is no guidancein the prior art as to how this property shall be implemented into aworking catalyst.

[0094] More detailed analysis of the results from the fixed-bed reactortests described in Examples 2 and 3 and other supporting tests indeedindicate that the cause of the selectivity improvement of the catalystaccording to the invention is associated with a reduced activity forhydrogenation of olefins although a simultaneous reduction in theactivity for termination of growing chains by hydrogenation can not beentirely excluded.

[0095] These conclusions are based on FIGS. 7 to 9. FIG. 7 shows propeneand propane selectivity as a function of support surface area forCo-Re/Al₂O₃ catalysts with particle size<100 microns (χ150·10¹⁶ m⁻¹).Co/Re=20-24, 5-20 wt % Co. These tests were fixed bed reactor tests at:210° C., 20 bar, H₂/CO=2.1, 45-55% conversion, about 100 h on stream.

[0096]FIG. 8 shows the effect of χ on propene selectivity using Al₂O₃supported cobalt catalysts with different surface area/α-aluminacontent. In this figure, open symbols represent high surface areaγ-alumina support; filled symbols represent low surface area γ-aluminasupport. These tests were fixed bed reactor tests at: 210° C., 20 bar,H₂/CO=2.1, 40-70% conversion, >24 h on stream.

[0097]FIG. 9 shows the effect of χ on propane selectivity using Al₂O₃supported cobalt catalysts with different surface area/α-aluminacontent. In this figure, open symbols represent high surface areaγ-alumina support; filled symbols represent low surface area α-aluminasupport. These tests were fixed bed reactor tests at: 210° C., 20 bar,H₂/CO=2.1, 40-70% conversion, >24 h on stream.

[0098] Thus, these Figures show decreased light paraffin selectivity forlow surface area/high α-alumina supported catalysts and indicate thatthe activity for olefin hydrogenation is reduced for catalysts accordingto the invention. (Propene/propane has been selected here asrepresentative of light olefin/paraffin products. Similar effects areobserved for other light products). FIG. 7 shows that although propeneselectivity is reduced for low surface area/high α-alumina supportedcatalysts, this is not accompanied by an increase in the production ofthe corresponding paraffin (propane).

[0099] A similar effect is observed when χ is increased by increasingparticle size (FIGS. 8 and 9). When χ is increased by increasing theparticle size, the olefin (propene) selectivity is continuouslydecreasing as a result of olefins being converted into secondaryproducts. Propane selectivity starts to increase at a χ-value of about1000×10¹⁶ m⁻¹, indicating that light olefins are converted to thecorresponding paraffin. This is a result of diffusion resistance on thereactants (H₂, CO) leading to low CO concentrations in the catalystpores and thus conditions more favorable for secondary olefinhydrogenation. Although the results show that this reaction can not beblocked totally for the catalysts according to the invention, thetendency for propane formation is lower for all χ-values.

[0100] Thus, the present invention describes a way of decreasing theolefin hydrogenation activity of a Fischer-Tropsch catalyst withoutsignificantly altering the main hyodrocarbon synthesis activity.

[0101] In addition to the indirect evidence described above, directevidence for reduced olefin hydrogenation activity for the catalystsaccording to the invention was found by performing separate olefinhydrogenation experiments. Selected catalysts prepared and pretreatedaccording to the procedures described in Examples 1 to 2 were tested ina fixed-bed reactor for propene hydrogenation activity. The results areshown in Table 8. The olefin hydrogenation rate for a low surfacearea/high α-alumina based catalyst is more than a factor of 2 lower thanthe catalysts included for comparison. TABLE 3 Propene hydrogenationactivity of 12% Co-0.5% Re/Al₂O₃ with different surface area and phasecomposition. T = 120° C., P = 1 atm. Feed consisting of 0.2 vol %propene, 1.3 vol % H₂ and balance He (diluent). Propane formationSurface area α - Al2O3 rate Cat. (m²/g) (%) (g/g cat./h) 2 191 0 1.1 366 7 1.1 4 16 86 0.4

1. A catalyst for use in a Fischer-Tropsch synthesis reaction whichcomprises cobalt supported on alumina, in which the alumina support hasa specific surface area of <50 m²/g.
 2. A catalyst for use in aFischer-Tropsch synthesis reaction which comprises cobalt supported onalumina, in which the alumina support is at least 10% alpha-alumina. 3.A catalyst as claimed in claim 1 and claim
 2. 4. A catalyst as claimedin any preceding claim, in which the specific surface are of the aluminais <30 m²/g.
 5. A catalyst as claimed in any of claims 2 to 4, in whichthe alumina is at least 50% and preferably at least 80% alpha-alumina.6. A catalyst as claimed in claim 5, in which the alumina issubstantially pure alpha-alumina.
 7. A catalyst as claimed in anypreceding claim, in which the cobalt represents from 3 to 35% by weightof the catalyst.
 8. A catalyst as claimed in claim 7, in which thecobalt represents from 5 to 20% by weight of the catalyst.
 9. A catalystas claimed in any preceding claim, further comprising a promoter.
 10. Acatalyst as claimed in claim 9, in which the promoter is rhenium,platinum, rhodium and/or iridium.
 11. A catalyst as claimed in claim 10,in which the promoter is rhenium and is present as 0.5 to 50% of thecobalt content.
 12. A catalyst as claimed in claim 10, in which thepromoter is platinum, rhodium and/or iridium and is present as 0.1 to50% of the cobalt content.
 13. A catalyst as claimed in claim 9,comprising up to 2% by weight of the promoter in total.
 14. A method ofmanufacturing a Fischer-Tropsch catalyst which comprises heat treatingalumina particles at a temperature in the range 700 to 1300° C. for aperiod of between 1 and 15 hours; and impregnating the heat treatedparticles with cobalt.
 15. A method as claimed in claim 14, furtherincluding the step of impregnating the alumina particles with cobalt,together with a promoter/dopant.
 16. A method as claimed in claim 15, inwhich the promoter is rhenium, platinum, iridium and/or rhodium.
 17. Theuse of a catalyst as claimed in any of claims 1 to 13 in aFischer-Tropsch synthesis reaction.
 18. A use as claimed in claim 17 inwhich the reaction is conducted in a slurry bubble column reactor.
 19. Amethod of converting natural gas to C₅+ hydrocarbons, which comprises;subjecting a natural gas feed stream to a reforming reaction to producea synthesis gas feed stream of hydrocarbon and carbon monoxide;subjecting the synthesis gas feed stream to a Fischer-Tropsch synthesisreaction in the presence of a catalyst as claimed in any of claims 1 to13; and separating a product stream including C₅+ hydrocarbons.